1. Field of the Invention
The present invention is directed to a method for measuring and characterizing the transfer function of a display device using only human perception as the measurement device.
The preferred embodiment is directed to measurement and characterization of a visual display device using only human visual perception as the measurement device. However, the present invention can be applied to the measurement and characterization of other display devices such as tactile or auditory display devices.
The characterization of a display device is typically used in a calibration system to correct the response of the display device to a known standard and/or to produce an application-specific profile that accurately describes the response of the display device.
In applications where visual display devices are utilized, such as the graphic design and publishing industry, a color calibration system is typically employed to produce ICC color profiles for output devices from their characterizations, these profiles later being used in a color management system to control color reproduction across these output devices.
Because this method makes no assumptions about a display device's characteristics, it can be utilized to measure and characterize any type of display device with any arbitrarily complex monotonic display transfer function. These display devices may include, but are not limited to emissive and reflective devices such as cathode ray tubes (CRT), backlit liquid crystal displays (LCD), reflective LCDs, transflective LCDs, gas plasma screens, digital light processing (DLP) devices and LED devices.
Until only recently, the cathode ray tube (CRT) has served as the most commonly used visual display device for desktop personal computers and television monitors. This is due to their combination of high image quality and relatively low price, and the lack of any other commercially viable display technologies that have competed with the CRT on these qualities. Portable computers have used alternate visual display technologies such as liquid crystal display (LCD) devices because of their small size, low weight and low power consumption, but their poor color quality, slow transient response and high price have made LCD technology unsuitable for desktop applications and professional graphic use. However, since their introduction, LCDs have improved dramatically in display quality and dropped in price to very affordable levels, and they are quickly becoming the new standard for desktop applications and entertainment use.
In the average business or home computer installation, the response of a visual display device is usually uncorrected. The brightness and contrast are typically set to some settings that are pleasing to the user, but no attempt is made to make the characteristics of the device match any known standards. While the display characteristics of this uncorrected display are naively acceptable to the average user, the accuracy of the display device is very critical to a graphic professional, and as a result, the display device needs to be calibrated to a known standard or characterized so that color management software can compensate for the display's characteristics. This is necessary so that the graphic artist can use the display to proof images that are targeted for another display device or reproduction medium before they are sent out for use in broadcast television or printed and distributed in literature, for example. If the images cannot be visually proofed before they are distributed, the resulting images may be quite different than what the artist had originally intended.
In the past when printed material and television were the primary distribution mediums for advertising and marketing materials, graphic professionals were the only users that absolutely required the use of a corrected display. However, with the proliferation of personal computers into businesses and households and the introduction of new display technologies like Gas Plasma and Digital Light Processing (DLP), new factors have emerged that necessitate the use of a corrected display by most users. These factors include:
Display technologies with characteristics that are different from that of CRTs display an image that looks very different than that same image viewed on a CRT. The vast majority of images available today were created on and targeted for the standard CRT, so when they are viewed on a variety of devices like LCDs and Gas Plasma displays, the images look very different than what the authors originally intended. Correcting a display's response helps minimize the differences between these different display technologies.
The advent of electronic commerce over the Internet has created a new medium for distribution of advertising and marketing materials and opened the doors to uncontrolled reproduction of potentially color-sensitive images. Traditionally, vendors of products, such as clothing, spend a great deal of time and effort insuring that the images printed in their catalogs are color correct and accurately reflect their product so that the user sees exactly what the delivered product will look like. However with the growth of electronic commerce over the Internet, many users are viewing these same images on web pages that are being viewed on uncorrected monitors. Each and every viewer may see a completely different rendition of the same product image, so it is impossible for a vendor to control what the viewer sees. Correcting a display's response helps maintain the fidelity between the appearance of the original image and the reproduced image.
As new types of display devices are introduced, buyers naturally have higher expectations for the qualities of these new devices, compared to the quality of the old devices that are being displaced. If these new devices perform differently than the displaced devices, these differences may be interpreted as poorer quality and hinder acceptance of the new technology. Correcting a display's response can help to eliminate the apparent differences between old and new technologies.
In order to correct the response of a visual display device, the relationship between the input signal and the perceived output from the face of the device must be known. Once known, this relationship can be used to re-map the input signal values so that the visual display device produces the desired output for a corresponding input signal value. There are numerous existing methods for re-mapping these values, whether it is done in the color look-up table of a video output card either via software or hardware, or in the hardware of a display device, but these methods are not the subject of the present invention.
This relationship between the input signal and the perceived output is called a transfer function. This function may be linear or non-linear, and is not required to follow any prescribed rules. For a CRT device, the relationship can be approximated by a simple power function, as illustrated in FIG. 2.
output=input^gamma
Where:
input=the input value, from 0 to 1 where 0=darkest desired output and 1=lightest desired output
output=the display output, from 0 to 1 where 0=darkest output and 1=lightest output
gamma=a constant
This power function states that the display output is equal to the input value raised to a constant exponential value, usually in the range of 2.3 to 2.6 for a typical uncorrected CRT.
This relationship exists in a CRT because of the physical relationship between the electrostatics of the cathode and the grid of an electron gun. When a phosphor in a CRT is illuminated by an electron gun, the amount of light given off (the output) is proportional to the applied input value (the input) as illustrated above. Most CRTs exhibit this behavior unless there is a design or manufacturing defect in the display, the display is very poorly adjusted, or the manufacturer has chosen to alter the display's behavior in some way.
In LCD devices, however, the relationship of input signal to perceived output is very different because LCDs operate on a different physical principle than do CRTs. Although LCDs vary from device to device, uncorrected LCDs typically have transfer functions that are shaped as illustrated in FIG. 3.
From the graphs of the transfer functions, one can see that CRTs and LCDs have very different response characteristics. An image displayed on an uncorrected CRT and an image displayed on an uncorrected LCD may look very different when viewed next to one another. Take the images in FIGS. 4, 5 and 6 as examples. The first image was created on a corrected CRT display with a gamma response of 1.8. Viewed on a CRT with a gamma response of 1.8, the viewer would see 11 distinguishable steps from black to white as illustrated in FIG. 4.
Viewed on a CRT with a gamma response of 2.5, the viewer would see an image that is darker overall than it should be. The two darkest patches appear to blend together, as illustrated in FIG. 5.
Viewed on an uncorrected LCD, the viewer would see an image that is too dark in the darkest region and too light in the lightest region. The two darkest patches appear to blend together and the two lightest patches appear to blend together, as illustrated in FIG. 6.
And just as LCDs differ from CRTs, other visual display technologies with different characteristics may produce equally different output images given the same input image.
Once the relationship between the input signal and the perceived output from the face of the device is known, the response of a display can be corrected and the detrimental problems mentioned previously can be reduced or eliminated.
The ability to correct the response of a display device allows any image created for that response to be properly displayed. A user would have the ability to adjust his or her display to suit the material being viewed, or in an ideal case, to have the computer do the adjustment automatically. For example, if images were being generated for print, the user could adjust the display to the response of the print medium and judge what the images would look like when printed. Similarly, if images were being generated for viewing on a web page, a user might adjust his display to the response of an uncorrected CRT to proof what the images would look like on the vast majority of the existing CRT displays connected to the Internet and viewing web pages.
2. Description of the Prior Art
There exist methods and devices for measuring the transfer function of a display device and subsequently correcting the response of the device through calibration.
Some of these methods involve attaching a hardware device to the display and measuring the actual output for every given input value. Methods such as this can be reasonably accurate, but they neglect certain factors like contributions from ambient lighting that will affect the accuracy of the measurements. In addition, a hardware device dramatically increases the cost of a solution, reducing its potential market.
Other methods are software-based, and rely on the user's ability to compare the brightness of two or more different patches displayed on the display device. Unfortunately, current methods are only applicable to CRT devices because they make the assumption that the response of the device follows the ideal gamma response of a CRT and that a single measurement is all that is necessary to determine the transfer function of the device. While this may produce an acceptable result some of the time, ambient lighting, design flaws and manufacturing defects can significantly alter the behavior of a display, rendering the results incorrect. When these methods are applied to a display device with different characteristics than a CRT, such as a LCD, the result is grossly inaccurate. To accurately determine the transfer function of a device, a plurality of measurements must be made across the dynamic range of the device in order to determine the relationship between input and output. No assumptions should be made about the shape of a particular device's transfer function.